1. Field of the Invention
The present invention relates to a water-based ink that comprises a water-insoluble coloring material. More specifically, the present invention relates to a water-based ink, which is preferably used for a recording method and a recording apparatus using an ink jet recording system, and further in an ink jet image forming method.
2. Related Background Art
It is known that ink containing a water-insoluble coloring material such as a pigment (pigmented ink) can provide images excellent in fastness such as water resistance or light resistance. In recent years, various techniques have been proposed to improve the optical density of an image formed with such ink. For example, there has been proposed a method for further improving image density, using an ink containing a self-dispersing carbon black and a certain salt (refer to, e.g., Japanese Unexamined Patent Publication No. 2000-198955). There has been also proposed a technique to form a high quality image, in which an ink composition for ink jet recording, containing a pigment, a polymer particle, a water-soluble organic solvent and water, is applied to a recording medium with an aqueous solution containing a polyvalent metal to cause reaction between the ink composition and the polyvalent metal in the aqueous solution (see e.g., Japanese Unexamined Patent Publication No. 2000-63719). In these techniques, a pigment present dispersed in the ink is forced to agglomerate on the surface of the recording medium to prevent penetration of the pigment into the recording medium whereby an image of a higher density than with conventional pigmented ink can be obtained.
However, as a result of intensive studies, the present inventors found out that, as the pigment particles are agglomerate on a recording medium in the above techniques, the area on the surface of a recording medium to be covered with the coloring material when a certain amount of the ink droplet is used (so-called area factor) tends to be insufficient. This means that the above techniques require more ink to obtain the same image density in comparison with the conventional pigmented ink in which a pigment is dispersed by a polymer dispersant or the like. This point needs further improvement. Although there is a method of obtaining a large area factor with a small ink droplet by increasing permeability of the ink to a recording medium, the ink of increased permeability diffuse not only on the surface of the recording medium but also into the inside thereof, resulting in insufficient image density.
The present inventors studied both advantages and disadvantages of the conventional inks and analyzed the characteristics of the images formed with such inks. They have found that, when an ink contains a coloring material in a larger amount, there arise such problems that coloring material remains in a larger amount on the surface of the recording medium, visually uneven dots are formed, or the coloring material is not effectively used but wasted in the recording medium. The present inventors have found that images superior to the conventional images can be obtained by solving at least one of these technical problems summarized below. The present invention solves at least one of the following problems.
(1) When a pigment which exists in the state of dispersion in an ink is forced to agglomerate on the surface of a recording medium, the area of the surface of the recording medium which the coloring material can cover with an ink droplet of a predetermined volume (so-called area factor) might be insufficient. In this case, the amount of ink necessary to obtain the same image density increases.
(2) When the ink permeability is increased, the ink spreads not only on the surface of a recording medium but also permeates in a direction of the thickness of the recording medium. Accordingly, the coloring material cannot distribute at a high density near the surface of the recording medium, and a high image density cannot be obtained.
Thus, an object of the present invention is to provide a water-based ink, which can provides sufficient area even with a small droplet of pigmented ink (large area factor), can provide an image having high OD (image density), and has a long-term storage stability.
Another object of the present invention is to provide an ink jet recording method of using the above ink to form a high-definition image with high OD with a small amount of the ink.
Another object of the present invention is to provide an ink cartridge, a recording unit, and an ink jet recording apparatus, which are preferably used for the above recording method.
Further, another object of the present invention is to provide an image forming method in which color mixing (bleeding) is effectively prevented on the border between a black ink region and a color ink region without causing feathering, when a color image in which different colors are adjacent to one another is recorded on a plain paper.
The technical concept of the present invention can be summarized as follows: a water-based ink that comprises water, a plurality of different water-soluble organic solvents, and a water-insoluble coloring material, wherein the above water-soluble organic solvents are a good solvent and a poor solvent for the water-insoluble coloring material to be used, and when Ka values of the above water-soluble organic solvents are determined by the Bristow method, the above poor solvent has the largest Ka value and diffuses along the surface of a recording medium in the form of substantially a perfect circle before the good solvent diffuses, assisting agglomeration of the above water-insoluble coloring material in the diffusion process. By this structure, the water-based ink of the present invention has an advantage that it is not necessary to contain a large amount of the coloring material that is diffused and wasted in a recording medium without contributing to image density as in the prior art. In addition, an image can be formed in an ideal state, that is, much of the coloring material is not located on the surface of a recording medium and at the same time the coloring material does not reach the back side of the recording medium thereby enabling printing on the both sides. As a result, an image with a high density is uniformly formed on the surface of the recording medium.